The invention relates to a heatable shoe for a joint welding apparatus for insertion between an underlying and an overlying portions of respective strips of a web material, such as asphalt roofing, to be joint welded through overlapping.
The invention has been developed in connection with the methodology which is used today when laying roofing on a base layer, where the roofing is rolled out on the roof base and laid with a certain interdependent overlapping. The general practice today for asphalt roofing is to heat the overlapping portions by means of a propane torch so that the overlapping portions are fused together. Hot air is also known to be used.
The types of hot air apparatus used today may deliver hot air with a temperature ranging from 20.degree. C. to 700.degree. C. The actual efficiency is poor and problems are frequently encountered in connection with electrical heating of the hot air at the building site (due to available fuses being too weak). Welding by means of a propane torch will yield temperatures that are higher than that of the hot air. Excessive heat supply will result in a fire hazard, since one has little or no control over the conditions underneath the overlapping web portions. Overheating may lead to melting and thereby destruction of heat-sensitive insulation, such as polystyrene. Under unfavourable conditions the temperature may be so high that the roof construction may ignite.
A proposal has also been made to make use of an electrically heatable shoe which is inserted between the underlying and overlying layers. With such a shoe, it has been difficult to achieve proper melting and welding. One reason is that a coking layer may easily form on the shoe. The cause for this is presumed to be that air or oxygen has relatively free access to the engaging surfaces between the asphalt roofing and the hot shoe. The coating destroys the desirable good heat contact within a short period of time.
The use of pressure means will to a large degree prevent this unfavourable coking and the coating resulting therefrom. The reason for this is presumed to be that air or oxygen to a large degree is prevented from entering the sensitive area between the shoe and the web material, since the pressure means will press the web material against the shoe and the close contact thereby achieved will to a large degree prevent coking or baking.
Use of a heatable shoe will also entail such great advantages, namely primarily good heat distribution and high efficiency, as well as elimination of the fire hazard, that it is desirable to use this special technique.
In the overlapping regions in asphalt roofing, the bottom underlying web is fastened to a respectively underlying substrate, e.g., a roof base by mechanical fastening means, and here it is particularly important to bring about an absolutely free flow of adhesive so that the mechanical fasteners can "drown", i.e., become embedded in the adhesive mass. Such "accumulation" of the adhesive, usually liquid asphalt, may to some extent be brought about through the use of a torch or hot air, but so far it has not been possible to bring this about through the use of a heatable shoe. Thus there is an obvious need for improvement of the shoe technique so as to arrive at a solution which not only has the same favourable characteristics as those of the torch/hot air technique, but which will be even better from an overall point of view.